The overall objective of the proposed research is to develop non-invasive optical devices for the detection and diagnosis of early Dental caries (dental decay). New, more sophisticated diagnostic tools are needed for the detection and characterization of caries lesions in the early stages of development. If carious lesions are detected early enough, they can be arrested/reversed by non-surgical means through fluoride therapy, anti- bacterial therapy, dietary changes, or by low intensity laser irradiation. The principal factor limiting optical imaging through the enamel of the tooth in the visible range of 400-700-nm is light scattering. Light scattering in sound enamel and dentin is sufficiently strong in the visible range to obscure light transmission through the tooth. The magnitude of light scattering in Dental enamel decreases markedly with increasing wavelength. Therefore, we posit that the near-IR region at 1310-nm offers the greatest potential for new optical imaging modalities due to the weak scattering and absorption in sound Dental hard tissue. At longer wavelengths, absorption of water in the tissue increases markedly reducing the penetration of IR light. The overall objective of this application will be achieved through the following Specific Aims: 1) to test the hypothesis that near-IR polarization-sensitive optical coherence tomography (PS-OCT) can be used for the accurate assessment of the depth and severity of natural occlusal lesions in vivo. 2) To test the hypothesis that a near-IR (NIR) imaging system operating at 1310-nm can be used to acquire high-contrast images of interproximal and occlusal caries lesions in vivo. It is likely that if these studies and future clinical trials are a success, that these novel technologies for imaging Dental hard tissue will be employed for the detection and monitoring of early carious lesions without the use of ionizing radiation, thereby enabling conservative non- surgical intervention and the preservation of healthy tissue structure. Moreover, it is probable that this proposed imaging technology would enable the clinician to detect and quantify the severity of occlusal lesions that are not resolvable with conventional radiography due to the surrounding sound tissue structure.